One challenge facing the delivery of radiation to treat pathological anatomies such as tumors or lesions is identifying the location of the target (i.e. tumor location within a patient). The most common technique currently used to identify and target a tumor location for treatment involves a diagnostic x-ray or fluoroscopy system to image the patient's body to detect the position of the tumor. This technique assumes that the tumor is stationary. Even if a patient is kept motionless, radiation treatment requires additional methods to account for movement due to respiration, in particular when treating a tumor located near the lungs. Breath hold and respiratory gating are two primary methods used to compensate for target movement during respiration while a patient is receiving conventional radiation treatments.
Breath hold requires the patient to hold his or her breath at the same point in the breathing cycle and only treats the tumor when the tumor is stationary. A respirometer is often used to measure the tidal volume and ensure the breath is being held at the same location in the breathing cycle during each irradiation. This method takes longer than a standard treatment and often requires training the patient to hold his or her breath in a repeatable manner.
Respiratory gating is the process of turning on the radiation beam as a function of a patient's breathing cycle. When using a respiratory gating technique, treatment is synchronized to the individual's breathing pattern, limiting the radiation beam delivery to only one specific part of the breathing cycle and targeting the tumor only when it is in the optimum range. This treatment method may be much quicker than the breath hold method but requires the patient to have many sessions of training to breathe in the same manner for long periods of time. This training requires many days of practice before treatment can begin. This system may also require healthy tissue to be irradiated before and after the tumor passes into view to ensure complete coverage of the tumor. This can add an additional margin of 5-10 mm on top of the margin normally used during treatment.
Attempts have been made to avoid the burdens placed on a patient from breath hold and respiratory gating techniques. In another method to track the movement of a tumor in real time during respiration, a combination of internal imaging markers and external position markers has been used to detect the movement of a tumor. In particular, fiducial markers are placed near a tumor to monitor the tumor location. The position of the fiducial markers is coordinated with the external position markers to track the movement of the tumor during respiration. External position markers are used because the fiducial markers are typically monitored with x-ray imaging. Because it may be unsafe to expose the patient continuously to x-rays to monitor the fiducials, the position of the markers can be used to predict the position of the fiducial markers between the longer periods of x-ray images. One type of external position markers integrates light emitting diodes (LEDs) into a vest that is worn by the patient. The flashing LEDs are then detected by a camera system to track movement.